For those that heat their rooms for tubs/racks how much do you spend a month?

Got our first PG&E bill back since I started heating the bedroom with a radiant oil heater and am not happy. Then again the boys are always opening the door and I'm always telling them to close it quickly behind them. And the apartment had no insulation in the walls. Tomorrow I'm going to stop by lowes and see about that widow plastic that you use the hairdryer to seal to stop window drafts.